This invention relates to composite mirror panels.
Composite mirror panels are useful for many purposes and an increasing use is being found in the field of solar energy collectors.
In one form of solar energy transducing installation, mirrors are held on supports distributed around a field. Each support may carry mirrors several square meters in area and there may for example be a hundred or more supports distributed around a field up to a few hectares in area. All the mirrors are arranged to reflect solar energy onto for example an energy collecting surface of a steam generator arranged to drive a turbo-generator. So that the mirrors can continue to reflect sunlight onto the energy collecting surface as the sun moves across the sky, the mirror supports incorporate means for adjusting the orientation of the mirrors about vertical and horizontal axes. The combination of such a mirror and support is known as a heliostat. Such mirrors may be flat or curved.
The efficiency and cost of such a solar energy transducing installation will depend on a number of factors, inter alia, the properties of the heliostat mirrors.
It is clearly desirable that a solar mirror, whether for use in a heliostat or some other form of solar energy collector, should have a high reflectivity, and that such reflectivity should be preserved against weathering to give the mirror a long useful life. It is also desirable that the mirror should be substantially rigid when in use. A heliostat mirror may, e.g. be located a hundred meters or more away from the collector, in which case even quite minor movement or deformation of the mirror due to wind gusts would deflect the reflected sunlight away from the collector surface.
The requisite properties of rigidity and weathering resistance can best be achieved by combining a mirror with a protective and strengthening backing means which together with the mirror forms a composite mirror panel. Composite mirror panels having such properties are useful not only for solar reflectors but also for other purposes, for example for use as building panels.
It is known, for example from U.S. Pat. No. 4,124,277, assigned to Martin Marietta Corporation, to form a composite concave mirror panel by holding a normally flat rigid glass mirror in a concave configuration under bending stress within its elastic limit by bonding it to a holding layer of substantially dimensionally stable material, the holding layer being formed by a layer of open-ended expansible cellular material sandwiched between layers of pliable and solidifiable dimensionally stable material. The holding layer is cured in situ and bonded to the glass mirror while the mirror is mechanically held in the required concave configuration. Aluminium and paper honeycomb structures are cited as examples of suitable cellular material. It is also known to form flat mirrors of the same composite structure.
Concave mirrors made in this way must be made so that each mirror has a predetermined curvature, within such manufacturing tolerances as may be allowed for their intended use. In the case of solar mirrors, such tolerances are very small. In the manufacture of curved mirrors comprising a reflectively-coated glass sheet held in flexed condition, the glass sheet is held pressed against a mould face or die of the required mirror curvature. It is therefore necessary when making concave mirrors of the aforesaid known construction to use a different mould or die for each different mirror curvature, even if such curvatures differ only slightly. When making mirrors for solar reflection purposes, eg for use as solar concentrators, mirrors of an appreciable number of different curvatures may be required.